This invention relates to semiconductor switching circuits and more particularly to such circuits utilized for power switching in direct current inverters.
In direct current inverter circuits, such as those found in DC link aircraft inverter power systems, power transistors are driven into saturation for switching output current to provide an alternating current power supply. For DC link inverter technology to be competitive with conventional aircraft power systems which utilize hydraulic constant speed drives coupled to synchronous AC generators to provide 400 Hz electrical power, the total power loss and circuit complexity must be held to a minimum while offering system performance equal to the conventional system.
This invention relates specifically to an improvement in transistor drive circuit technology which improves switching time of power transistors while limiting transistor power dissipation without significantly increasing circuit complexity.
DC link power inverters have used a transistor drive concept known as current-coupled feedback in which a transformer is used to deliver a portion of a switching transistor's collector current to the transistor base circuit. An inverter using this concept to control switching transistors is disclosed in U.S. Pat. No. 3,715,648 issued to Kernick et al. on Feb. 6, 1973. Using the current-coupled feedback technique, base current is always proportional to collector current, the current ratio being set by the transformer turns ratio. This turns ratio is determined from the minimum gain of the transistor which typically occurs at the peak collector current. Therefore, adequate base current is supplied at all loads without having to provide maximum base current at all times. Even though the maximum base current is not delivered at all loads, a base current in excess of the transistor requirements at nearly all loads is normally applied. This occurs because the transistor current gain is typically much higher at reduced current levels than at the maximum rated current.
To avoid problems associated with excessive base current drive, various circuits have been utilized to divert excessive base current. Several of these circuits are described in U.S. Pat. No. 4,246,501 issued to Baker on Jan. 20, 1981. Such circuits generally result in an increase in saturation voltage level of the transistor in order to achieve the desired current diversion. This increased saturation voltage leads to undesirable increases in transistor power dissipation.
A transistor switching circuit constructed in accordance with the present invention utilizes a branch circuit which includes the series connection of a first and second diode. This branch circuit is connected to the base of a switching transistor; a third diode is connected between the junction of said first and second diodes and the collector of the switching transistor; and a capacitor is connected in parallel with the branch circuit. This circuit arrangement provides for diversion of excess transistor base current from the base circuit to the transistor collector, while minimizing any increase in transistor saturation voltage.